Two-way Halo Implant
A system and method for ion implantation during semiconductor fabrication. An integrated circuit may be designed with proximately located one-directional transistor gates. A two-way halo ion implantation is performed perpendicularly to the transistor gates in order to embed the dopant into the silicon body on the surface of the semiconductor wafer. The two-way halo both reduces the channeling effect by allowing ion implantation beneath the transistor gate, and reduces the halo shadowing effect resulting from halo implanting which is done parallel to the transistor gates.
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The present application is a divisional of U.S. application Ser. No. 12/396,855, filed Mar. 3, 2009, entitled “Two-Way Halo Implant,” which is a non-provisional of U.S. Provisional Application Ser. No. 61/034,329, filed Mar. 6, 2008, entitled “Two-Way Halo Implant,” the contents of which are incorporated herein by reference in their entirety for all purposes.
TECHNICAL FIELDAspects of the present invention are generally directed to semiconductor devices and their methods for manufacture, and more particularly to providing a way to dope regions of a transistor while reducing the shadowing effect of angled ion implantation.
BACKGROUNDIon implantation, also known as doping, is one of the key technologies in the fabrication of semiconductor integrated circuit devices. With current ion implanters, a very accurate and extremely pure dose of a desired atomic species can be introduced into a target material. For example, boron ions may be directed from an ion source towards a semiconductor substrate with an accelerator, and may penetrate the silicon body of a semiconductor surface at a desired dosage level.
Angled ion implantation, also known as tilted or “halo” implantation, involves embedding ions into a material at the semiconductor surface at an angle that is not normal to the semiconductor surface. Halo implants are commonly used in semiconductor fabrication to control the short channel effect under transistor gates or other masking structures formed on the semiconductor surface, which may disrupt the ion implantation to the semiconductor surface regions beneath the transistor gate or other structure. However, while a halo implantation may control the short channel effect, such an implantation may produce an implant “shadow” near the gate on the side opposite the ion source, where implantation is obstructed within an area shadowed by the gate. The location and size of the shadow area depends on the direction of the implant, the implant angle, and the height and profile of the transistor gate. This shadowing effect limits the ability to distribute ions at the proper locations and at the proper dosage level into the semiconductor surface, and may limit size reduction and design flexibility during semiconductor fabrication.
SUMMARYIn light of the foregoing background, there is a need to reduce the shadowing effect resulting from angled ion implantations, thereby possibly permitting size reductions and increasing device design flexibility during semiconductor fabrication. In one aspect of the present disclosure, a two-way halo ion implantation may be performed on an integrated circuit. Each ion implantation may direct ions of a particular composition downward onto and/or into a silicon layer surface of the semiconductor device, at an angle not normal to the surface. The implantation may be on opposing sides of a long axis of one or more transistor gates form on the semiconductor surface.
According to another aspect of the present disclosure, the two-way halo implants may be configured perpendicular to the transistor gates to direct ion implantation onto and/or into regions of the silicon layer beneath the gates, thereby potentially controlling the short channel effect under the transistor gates. According to yet another aspect of the present disclosure, the circuit, such as an SRAM, may be designed with proximately located one-directional transistor gates, so that certain halo implanting parallel to those gates may be unnecessary. Thus, shadowing effect of transistor gates may be effectively reduced, thereby potentially lowering the required ion concentration in certain portions of the silicon layer, along with the ion concentration differential between adjacent regions resulting from gate shadowing.
These and other aspects of the disclosure will be apparent upon consideration of the following detailed description of illustrative embodiments.
Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
Illustrative embodiments will now be described more fully with reference to the accompanying drawings. The embodiments set forth herein should not be viewed as limiting; rather, these embodiments are provided merely as examples of the concepts described herein.
It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect.
These ion penetration regions 308-313 illustrate the shadowing effect of transistor gate 203 when using four-way halo implantation. The large number of ion penetration regions 308-313, the large differential in dosage among regions, and the high overall implant dosage in some regions, may cause threshold voltage scattering within a transistor and threshold voltage mismatch between multiple transistors in the same circuit. For example, a static random access memory (SRAM) may use multiple transistors positioned closely together, making the SRAM particularly vulnerable to the shadowing effects of a four-way halo. A high implant dosage may also require the use of a high external resistance, and may cause reverse junction leakage at the gate edge, which may in turn increase the reverse junction current to forward junction current ratio (IR/IF) of the transistor.
A more detailed description of illustrative devices and manufacturing processes will now be discussed in connection with
Two adjacent conductive transistor gates 403 are 404 (e.g., polysilicon) are formed on the silicon body 402. Transistor gates 403 and 404 may be part of, for example, an n-type field-effect transistor (NFET) 450 adjacent to a p-type field-effect transistor (PFET) 451, both of which are formed in a conventional manner in and on the silicon body 402 on opposing sides of STI layer 405. Transistor gates 403 and 404 may be disposed on silicon body 402 with a thin gate oxide layer (not shown) between the gates 403, 404 and the silicon body 402. In addition, transistor gates 403 and 404 may be re-oxidized in a conventional manner, resulting in an approximately 5 nm wide re-oxidation layer, not shown, on the sidewalls of gates 403 and 404.
Caps 407 and 409 may be formed on top of transistor gates 403 and 404, respectively. Caps 407 and 409 may be formed, for example, by depositing a layer of SiN on top of gates 403 and 404, so that each cap 407 and 409 has a thickness of approximately 50 nm thick or less. Sidewall spacers 406 and 408 may be formed on both sides of gates 403 and 404 and on silicon body 402, and may be formed differently depending on the gate type. For example, mask layer 406 may be a second SiN layer deposited over NFET gate 403, while conventional reactive-ion etching (RIE) may be performed using patterned photo resist layer as a mask to result in spacers 408 on the sidewalls of the PFET gate 404. Sidewall spacers 406 and 408 may each be, for example, approximately 40 nm in width. Doped source/drain regions 410 and 411 may be embedded in the silicon body 402.
Referring to
Referring again to
To illustrate the reduced shadowing effect in this embodiment, halo implant 605 directs ions into the silicon body 402, and specifically into regions 708, 709, and 711. However, since region 710 is hidden, or blocked, by the transistor gate 403, ions from halo implant 605 may not reach this region in any significant quantity. Similarly, halo implant 607 may deliver ions into silicon body regions 708, 709, and 710, but region 711 may be shadowed from halo implant 607 by the transistor gate 403.
Thus, in this example, the two-way implantation from halo implants 605 and 607 into the silicon body 402 near the transistor gate 403, results in only four distinct ion penetration regions 708-711. Regions 708 and 709 contain ions from both halo implants 605 and 607. Region 710 contains embedded ions from halo implant 607, but is in the shadow area of halo implant 605, and therefore might not contain ions from halo implant 605. Similarly, region 709 is shadowed from halo implant 607, but does contain embedded ions from halo implant 605. Thus, regions 710 and 711 may have a lower ion concentration as a result of the implant dosage than regions 708 and 709, but may have the similar or same ion concentration as each other. For instance, where the dosages of halo implants 605 and 607 are equal to each other, regions 710 and 711 each may have approximately half the ion concentration as each of regions 708 and 709.
As illustrated by
Additionally, the reduction in implant dosage may allow for a corresponding reduction in external resistance, without necessarily requiring a change in the threshold voltage of the transistor. Further, when a two-way halo implant is applied to a SOI field-effect transistor, reverse junction leakage may be reduced at the gate edge as a result of the lower well implant dosage. This reduction may improve the IR/IF ratio of the transistor.
A potential benefit from a two-way halo implant relates to the use of chemical resist in semiconductor microlithography. During the lithographic process, a layer of photosensitive chemical resist may be applied to a surface of the semiconductor wafer. The wafer surface may then be exposed to a form of radiant energy, such as ultraviolet light projected through a mask onto the surface of the wafer, resulting in physical or chemical changes to the exposed resist layer. The surface may then be rinsed with an appropriate substance, such as a chemical solvent, to form a conductive layer in a desired image on the semiconductor surface. The two-way implant may take place after the application of the resist but before the undeveloped resist is removed with the solvent rinse.
While the foregoing descriptions and the associated drawings may relate to a semiconductor fabrication process, many modifications and other embodiments will come to mind to one skilled in the art having the benefit of the teachings presented. The illustrative embodiments described herein may be adaptable to any manufacturing process that uses a particle implantation into a material.
Claims
1. A method for manufacturing a semiconductor device, comprising:
- forming a conductive layer on a top surface of a silicon layer, the conductive layer having a width and a length, wherein the length of the conductive layer extends along an axis and is longer than the respective width;
- forming a layer of resist on said conductive layer;
- directing a first ion stream toward the silicon layer at a first angle not normal to the top surface of the silicon layer, said first angle being perpendicular to the axis;
- directing a second ion stream toward the silicon layer at a second angle not normal to the top surface of the silicon layer, said second angle being perpendicular to the axis, wherein said second ion stream originates from an opposite side of the axis from which the first ion stream originates; and
- removing at least some of said layer of resist from said conductive layer, wherein only two ion streams are directed toward the silicon layer between steps of forming said layer of resist and removing at least of the said layer of resist.
2. The method of claim 1, wherein the top surface of the silicon layer is not doped by an ion stream directed at an angle parallel to the axis.
3. The method of claim 1, wherein the first and second ion streams are each boron ion streams.
4. The method of claim 1, wherein the conductive layer comprises a plurality of regions of polysilicon.
5. The method of claim 4, further comprising:
- forming an SRAM comprising a plurality of transistor gates, wherein said plurality of regions of polysilicon comprise said transistor gates of said SRAM.
6. The method of claim 5, wherein said first ion stream and said second ion stream are directed toward the silicon layer at an angle based on the height one or more of the plurality of transistor gates and a distance between two or more of the plurality of transistor gates.
Type: Application
Filed: Jun 28, 2011
Publication Date: Oct 20, 2011
Applicant: KABUSHIKI KAISHA TOSHIBA (Tokyo)
Inventor: Katsura Miyashita (Naka-gun)
Application Number: 13/170,370
International Classification: H01L 21/8232 (20060101);